1. Field of the Invention
This invention relates, in general, to apparatus for scanning lines of original documents and, more specifically, to charge coupled devices suitable for color scanning.
2. Description of the Prior Art
Document scanners are used with the various types of equipment to obtain image data from an original document or picture. These scanners use electronic elements to convert the light signals into electrical signals suitable for storage in a solid state memory. Charge coupled devices (CCD's) are frequently used as the basic optical sensor element in such scanners. The CCD is supplemented by additional electronic circuits to control and synchronize the CCD data with the scanned document.
In order to use scanners in applications where the color of the scanned document is to be detected or recognized, special sensors must be used to provide the color information. While exposing the document with different color light and using a conventional monochromatic CCD does provide the information needed, the preferred arrangement is to expose the document with white light and filter the color components received by the sensor. Such filtering is conventionally provided by colored filters positioned directly over the photosensitive sites of the sensor device. In some applications, however, diffraction of the imaged light is used to position different color components at different locations and sense them independently with separate photosensitive sites or with the same site displaced in location by the movement of the scanning device during operation. Even other forms of color sensing have been used according to the prior art. Regardless of which method of sensing color images is used, the data from the sensor or CCD must be able to distinguish relative amounts of light energy for different color components of the image from the document.
When image light directed to a CCD sensor is filtered before impinging upon the photosensitive sites, or photosites, the filters are usually placed over specific photosites by use of a mask during construction. The mask restricts the application of the filters to only those photosites which are to be covered with that color filter. Since CCD's often have several thousand photosites and three color filters are usually used, the process of masking off parts of the CCD for each color filter is rather tedious and is susceptible to error and defects. This causes the yield of acceptably filtered CCD's to be lower than desirable. When something in the masking process goes wrong or otherwise does not turn out as desired, the complete operation must be performed over again, possibly at the loss of a complete CCD sensor.
Gelatin filters are frequently used according to the prior art and are reasonably easy to apply. However, gelatin filters have a very wide bandwidth and are not suitable for many applications. Another process which does produce suitable bandwidth for many applications is known as metal deposition of interference filters. This process requires masking for each color, and each color may be made of 20 or 30 different depositions. Thus, the yield is potentially poor with metal deposition of filters onto CCD's. Therefore, it is desirable and it is an object of this invention to provide a filter arrangement for light sensors which is easy to apply and which facilitates high yields of acceptably filtered CCD's.
CCD's are available as both area sensors and as line sensors. Area sensors are capable of sensing a two-dimensional image without relative movement between the sensor and the original document. Line sensors are only able to scan entire two-dimensional documents by moving the lens sensor and document relative to one another. Line sensors which are used for monochromatic image sensing have a single row of photosites which correspond in number to the pixel resolution of the sensed image data. To maintain the same pixel resolution, a color sensor with filters would need three such line sensors arranged in parallel with separate color filters.
U.S. Pat. No. 4,763,189, issued on Aug. 9, 1988, illustrates a color sensor constructed according to the prior art. That patent pertains to a method of constructing the three parallel line sensors and their respective color filters in such a manner that the interconnections between the sensor arrays and the output circuitry can be made easier, consistent with high resolution and speed. However, in many applications, the resolution provided by three line sensors in parallel is not required. This requires that every third or fourth photosite in the same line of sensors have the same color filter, and this can be provided with the prior art teachings which mask the photosites for different colors. This may lower the pixel resolution, but line sensors of very high resolution are available to compensate for the resolution decrease with such a filter system. When the resolution provided by a single line color sensor (no overlapping of the photosites as the lines are scanned) is sufficient, the present invention provides a unique way to position the filters over the photosites.